Categorising bacteria in purple and pink

When confronted with a new bacteria there are a series of simple tests that can be carried out to give a rough idea of the properties of the bacteria you are dealing with. One of the simplest and most useful tests is known as “Gram staining” which is a process of staining cells either purple or pink depending on the properties of their cell walls. It was discovered by Christian Gram who wasn’t initially trying to find a test for bacteria, he just wanted to make them visible.

Gram staining colours the bacteria either purple, in which case they are referred to as “Gram positive,” or pink which are known as “Gram negative”. Although Christian Gram didn’t realise it at the time, this different response to the staining technique is due to a fundamental and important change in the structure of bacteria. Gram positive cells have large bulky cell walls made out of a substance called peptidoglycan. In contract Gram negative bacteria have two thin cell membranes with a thin peptidoglycan layer between them.